In photonics since 1999. Benelux technical support, answer within one working day.
What an achromatic doublet actually corrects
An achromat cements two glasses with different dispersions so that two wavelengths share a focus. It removes most longitudinal chromatic aberration and, as a side effect, reduces spherical aberration. It does not make the lens perfect: a third wavelength still lands slightly elsewhere.
THE SHORT ANSWER
An achromat cements two glasses with different dispersions so that two wavelengths share a focus. It removes most longitudinal chromatic aberration and, as a side effect, reduces spherical aberration. It does not make the lens perfect: a third wavelength still lands slightly elsewhere.
| Elements | 2, cemented |
|---|---|
| Corrected at | Two wavelengths |
| Typical coating | VIS-NIR AR, 400-1000 nm |
| Surface figure | Lambda/4 at 632,8 nm |
| Centration | Below 3 arcmin |
| Residual | Secondary spectrum |
Glass bends blue light more than red. Put a single lens in a white beam and the blue focus sits closer to the lens than the red one, which is why a singlet images a bright edge with a coloured fringe.
The two-glass trick
An achromat cements a positive element in a low-dispersion glass to a negative element in a high-dispersion glass. The powers are chosen so the two elements together still converge the beam, but their dispersions cancel at two chosen wavelengths. Those two wavelengths now share a focal plane.
The useful accident is that the same design freedom used to cancel dispersion also reduces spherical aberration. An achromatic doublet almost always outperforms a singlet of the same focal length on axis, even in monochromatic light.
What remains
Two wavelengths are corrected, not all of them. A third lands slightly short or long, and that residual is the secondary spectrum. For most laboratory imaging it is invisible; for precision metrology across a wide band it is the limit, and the answer is an apochromat or a reflective design.
Specifying one honestly
State the wavelength band first, then the aperture, then the focal length. The band determines the coating and whether the standard visible correction is appropriate at all. A doublet ordered on focal length alone frequently arrives correct and useless.
When to use it
- Broadband or white-light imaging where a singlet shows colour fringing
- Collimating or focusing a source with real spectral width
- Relay trains where a singlet's spherical aberration would accumulate
- Any imaging job with more than one wavelength in it
What to watch out for
- A doublet corrected for the visible is not corrected at 1064 nm; match the coating and the correction to your band
- The cement is the weak point under high power: check the damage threshold before using one in a pulsed beam
- Focal length is specified at a design wavelength and drifts across the band
- Achromat does not mean aberration-free; off-axis performance still needs checking
Still deciding?
Tell us the wavelength, the aperture and what you are measuring. We answer with a part number, a price and a lead time, usually the same working day.